The ESRB has released its rating and comments for Duke Nukem Forever. If you had any doubts as to whether this is going to be a proper sequel, just check out this bit:
http://www.esrb.org/ratings/synopsis.jsp?Certificate=30650
A few sequences strongly imply sexual acts: two women appear to perform fellatio on the central character (e.g., raising their heads from his lap)—though there is no actual depiction of the sex act; Duke moans and leans up against the wall of a bathroom stall (furnished with a hole)—it is implied that a sex act (not depicted) occurs.
http://www.esrb.org/ratings/synopsis.jsp?Certificate=30650